dr. d.c. hopkins, phdabcs of power electronic systems2- 1 by dr. douglas c. hopkins

Dr. D.C. Hopkins, PhD ABCs of Power Electronic Systems 2- 1

By

Dr. Douglas C. Hopkins


Specifications• Right is a power supply with

inputs and outputs

• Electrically transfers power from the source to the load

• Mechanically has size and operates in a certain thermal environment

• So the power supply spec’d into two sections

– Electrical

– Mechanical

• The electrical specifications can be split up as follows

PowerSupply

Iin

Vin

Iout

Vout

Source Load


Electrical Specs

V in , I in

A C

V in ,I in

D C

In p u t

V ou t, Iou t

O u tp u t

P G ood , O n /O ffe tc

C on tro ls

E ffic ien cy

M isc

E lec trica l S p ec


Input Specs

• The specifications of the input will depend on the source whether its

– DC

– AC

• Cover first specifying the input specs of a power supply running off a DC source


DC Input Spec

• Specifying the Vin for a PS (Power Supply) will depend on the DC source voltage range

• A DC voltage is described as a voltage that will very slowly over time or is a constant

• Here are some typical DC sources with their voltage range– Car Battery typical 12V with 11 to 14 volts variation

– Solar Cell 0.5 to 1 volt per cell depending on sunlight

– Telecom Bus typical 12V with 36 to 72 volts variation

– PC Internal 5V Bus 5V +/-10%

• The range for Vin should be chosen such that it covers the DC source it intends to run off


DC Input Spec

• For a PS that runs off a Telecom bus would need to have a Vin operating range of 36 to 72 volts

• If the input voltage drops below 36V, typically a PS will shut down

• If the input voltage exceeds 72V, typically a PS will be damaged by the excessive high voltage

– A PS can be designed such it can handle short duration of high input voltage such as line transients due to lightning

– This is known as surge rating

– For example this PS can have a surge rating of 100V for 100usec.


DC Input Spec

• The current drawn by the PS is known as Iin and measured in amps

• Iin is derived by this relationship– Output power (Pout) of a PS is Vout x Iout

– Input power (Pin) of a PS is Vin x Iin is the efficiency of the PS

• Describes how much of the input power results to the output

• typically between 0.5 to 0.95

• This will be covered in more detail later

Pout = Pin

– Substituting and solving for Iin

– Iin = (Vout x Iout) / (Vin x )


DC Input Spec– Iin = (Vout x Iout) / (Vin x )

• Note that if the output and is held constant, Iin will vary 1/Vin

• Worst case Iin occurs at lowest value of Vin

• For our example, the PS will draw the most current at Vin=36 volts


DC Input Spec• Ideally Iin should be a clean DC

current

• But there will be the ripple current, Irip, from the switching stage within the PS itself

• Usually spec’d as a peak to peak current

• The frequency of this waveform is usually < 10Mhz

• Typically this is < 10% of maximum Iin

• In our example, If Iin max is 10A, Irip p-p should < 1A

IripIin

Time


DC Input Spec• There will be noise that occurs at >

10Mhz.

• Known as switching noise and its due to the internal parasitics

• Typically the peak to peak noise is less than 1% of max Iin

• Finally there is surge current

• This is due to Vin rising quickly and charging int

Time

Iin

Time

Iin Vin


DC Input Spec

• Finally there is surge current

• This is due to Vin rising quickly and charging internal capacitors in the PS

• Once the capacitors are charged, Iin will approach its steady state operation

• Usually Isurge is less than 5 times max Iin

Time

Iin Vin


AC Input Spec

• The other form of input for PS is AC

• The source for this is usually AC generators from the big utilities

• There are two types of transmission of AC utility– Single Phase

– Three Phase

• Three phase is used for systems >5KW

• This will not be covered in this course

• The main reason AC is used for transmission of power is because voltages can be easily stepped up and down with the use of transformers


AC Input Spec

• Some definitions of Vin for AC• In discussing Vin, Vpk is rarely used• Instead the “DC” equivalent value

known as RMS is used• RMS stands for Root Mean Square and

the input voltage would be labeled as Vin-rms

• Vin-rms = Vpk / 1.4142 or Vpk/(2)0.5

• Because the voltage is not constant, RMS some makes calculations easier

– For DC, Pin = Vin x Iin– For AC, Pin = Vin-rms x Iin-rms– Valid if Iin is a sine wave

Time

Voltage

Vpk


AC Input Spec

• Here are some typical AC utility voltage ranges for the home– Doorbell, heating systems 24Vrms +/-30%

– Household wiring Typically 110Vrms with 90 to 130 range

– Electric stoves Typically 220Vrms with 180 to 260 range• Actually 220Vrms with a center-tap is delivered to the home. 110Vrms is

derived from the center-tap

• Here are some typical AC utility voltage ranges for the business with the AC single phase derived from three phase

– Office wiring Typically 120Vrms with 90 to 140 range

– Inustrial/Computer Typically 208Vrms with 180 to 260 range

• Smaller businesses will use the household AC utility


AC Input Spec

• Europe and some other countries do not use 110Vrms or 120Vrms

• All homes and businesses are wired with either 208Vrms or 220Vrms

• There are several reasons why this voltage will very– Voltage drops due to the transmission of power

• IE a person living near a sub-station may have typically 130Vrms while someone 20 miles away may have only 110Vrms

– The more current drawn on the wiring more voltage drop• IE turn on a vacuum cleaner in a house and the lights will dim slightly

– Voltage can increase temporarily because the load is suddenly removed and the generators have not responded to the change


AC Input Spec

• Here are some spec’s for Vin for typical products– Desktop PC sold in the US, 90Vrms – 140Vrms

– Desktop PC sold in the Worldwide, 180Vrms – 260Vrms

– High end server sold worldwide, 180Vrms – 260Vrms

– Desktop PC with “universal” PS sold worldwide, 90Vrms – 260Vrms

• Why not use a “universal” PS in all desktop PC’s ?

• “Universal” PS are more expensive and difficult to design


AC Input Spec

• Another spec for Vin is the frequency

• Frequency is defined as one over the time it takes to complete one whole cycle

• In the use we use 60Hz

• In Europe and some other countries, they use 50Hz

• The range is only +/-3Hz

• A “universal” PS operates from 47Hz to 63Hz

• This is not a cost or a design problem

Time

Voltage

Frequency=1/time

For a complete cycle


AC Input Spec• Like DC, AC utility will have voltage surges

– These typically be quite large– 2000V spikes are no uncommon

• Florida is the worst US state

– They are due to lightning, some industrial equipment and solar flares– The front-end circuitry must be able to shunt this energy

• Since the AC utility voltages are much higher than DC and there is nothing to limit the current, the PS have no direct connection between input and output

• This is known as isolation• This is a safety requirement• More will be discussed later


AC Input Spec

• Another problem with AC utility is brown outs, sags or drop outs in the power

• This is when voltage drops below its specified Vin range

• This occurs if there are problems with the utility grid – Transformer in a sub-station goes bad

– Too much load on the grid like air-conditioners in the summer

– Solar flares can induce too much voltage and “pop” the breakers

• These occur quite often– How many times have the lights “flickered” ?

– That was a drop out

• More than 99% of the drop outs are less than 20ms in length


AC Input Spec

• PS in most products will just shut down temporarily during these events

– Televisions, radios, VCRs

– Non mission critical devices

• In mission critical devices such as high-end servers, the PS shall maintain operation for a loss of AC line up to 20ms

• After 20ms it can shut down

• This is known as hold-up time

• This is accomplished by a large energy storage device like a capacitor

• Typical specifications for hold-up is 20ms


AC Input Spec• Ideally Iin should follow Vin as

shown on the right

• This is known as emulating a resistor

Time

Vin

Iin


AC Input Spec• Usually for a PS, the input is a bridge

diode rectifier with a large capacitance

• Iin with respect to Vin will be distorted

• In both cases the input power is the same

• However, Iin-rms is now significantly higher

• Pin=Vin-rms x Iin-rms is the apparent power

• The real power is the average of (Vin x Iin)

Time

Vin

Iin


AC Input Spec

• The term to describe this Power Factor, PF

• PF = Real Power / Apparent Power

• The lower the PF, the higher the Iin-rms for the given power

• The problems with lower PF are– Wire sizes must be increased to handle the higher Iin-rms current

because Power Loss = resistance of the copper x (Iin-rms)2

– This is extra power utilities must size for and supply

– Iin is rich in harmonics which adds noise and circulating currents in 3 phase systems


AC Input Spec

• Another criteria is the Total Harmonic Distortion, THD

• Any waveform can be broken down into a sum of sine waves with different amplitudes

• A pure 60Hz signal should only be made up of a 60Hz signal

• If there is any distortion, then– I=I1sin(2ft)+I2sin(4ft)+I2sin(6ft)+…….– I1 is the root-mean-square (rms) of the fundamental current waveform– I2 is the rms of the second-order harmonic current waveform– I3 is the rms of the third-order harmonic current waveform, etc.

• THD = [square root ((I2)^2 + (I3)^2 + (I4)^2 + ...)/ square root (I1)^2] x 100 (to convert to percentage)


AC Input Spec

• Any distortion will show up with THD > 0%• A good value for THD < 5%• Will discuss these regulations later

Reference http://www.lrc.rpi.edu/NLPIP/Online/Answers/LA-Power/Sidebars.htm


AC Input Spec

• Most PS with a simple diode bridge and capacitor has a PF0.67 and THD20%

• The above PS with a large inductor in series can achieve a PF0.9 but still THD20%

• A Pwer Factor Correction circuit used in a front-end PS can achieve PF>0.98 and THD<5%

• However a PFC adds cost complexity to the PS


AC Input Spec

• Noise on Iin is another issue• Its not measured the same way as DC inputs• Since these PS connect directly on the grid, this noise can

propagate• If this noise if high enough in level it can effect equipment on

the grid significantly• If the frequency is high enough, it can use the external wiring

as an antennae


AC Input Spec

• If the frequency is less than 30Mhz, this is known as conducted

– This is measured with a box called a “LISN” which filters out the 50/60hz

– This is connected to the line cord– A frequency spectrum analyzer will then display the noise spectrum

• If the frequency is greater than 30Mhz, this is known as radiated

– This is measured with an antennae usually 10 meters away– At these frequencies, line cords and cables become very effective

antennae• There are federal specifications that must be met• This will be discussed later


AC Input Spec

• Like DC, Iin has inrush issues with AC applications• Usually peak Iin is spec’d to be less than 5Xof the steady state

of Iin-rms• Another factor to take account is fusing and circuit breakers• If the inrush current is too high or can occur throughout the

day, fuses and circuit breakers can be weakened, damaged or open up

dr. d.c. hopkins, phdabcs of power electronic systems2- 1 by dr. douglas c. hopkins

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